A known technique of maximizing the use of radio spectrum in multi-site radio communication systems is frequency reuse. In cellular systems, for example, frequencies are reused by arranging transmitter/receiver sites into reuse clusters, each of which establishes simultaneous links to subscriber units on different frequencies. This typically requires at least three duplex radio channels (each channel having two carrier frequencies, one inbound and one outbound), with some systems using more. In some wide area data radio communication systems, frequency reuse is also employed. A technique for reuse in a single duplex channel, multi-site data radio communication system is described in U.S. Pat. No. 4,670,906, entitled "Data Communication System Transmitter Selection Method and Apparatus" by Thro, issued Jun. 2, 1987. In the multi-site data radio communication system described in the patent, the transmitters have a fixed power output and when multiple messages are scheduled for simultaneous transmission, some transmitters are typically not usable because co-channel interference is likely. Also, messages are scheduled one at a time, in the order in which they are accepted for delivery. While this approach to frequency reuse exhibits improved message throughput over systems which do not employ frequency reuse techniques, the efficiency of frequency reuse could be higher if multiple messages could be scheduled for transmission from more transmitters.
Thus, what is needed is an improved technique for scheduling multiple messages for simultaneous, frequency reuse transmission from multiple transmitter sites in a multi-site data radio communication system.